Materials that serve above about 1000°F (540°C). In the broad sense, high-temperature materials can be identified by the following classes of construction solids: stainless steel (limited), austenitic superalloys, refractory metals, ceramics and ceramic composites, metal-matrix composites, and graphitic composites. The first three classes are well proven in industrial use, although stainless steels serve but slightly above 1000°F (540°C) and refractory metals are usually limited to nonoxidizing atmospheric conditions. The other classes are under extensive worldwide research to establish whether they can be utilized to replace and extend the capabilities of austenitic superalloys, which are the mainstay of high-temperature service.